Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE OF THE INVENTION
PROCESS FOR SUPPLYING NITROGEN
FROM AN ON-SITE PLANT
BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to a process for supplying
nitrogen from an on-site plant.
Description of the Backqround:
For a wide variety of industrial plants, it would be quite
advantageous to produce nitrogen on-site. For example, it
would be quite useful to produce a nitrogen stream on-site from
chemical plants or steel plants where blanketing with an inert
gas stream is often required, or in pulp and paper mills, where
it is often necessary to pad chemical rail cars or tanks w\ith
nitrogen. In those plants where a huge amount of nitrogen is
necessary, it is generally cheaper to connect the plant to a
nitrogen pipeline fed by a nitrogen plant which collld be remote
from the said plants.
When the requirements are not too important, the customer
usually has a tank which is regularly filled up with liguid or
bulk nitrogen. This solution might be sometime expensive if
only a small amount of gas is consumed. There are also cases
where the purity required by the customer is either less than
that o~ the bulk product or, on the contrary, is higher than
that of the bulk product, e.g. for high technologies purposes
such as semi-conductor industry or the like. Ther is
presently no simple and cheap solution to achieve that goal~
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In addition to the above, there is a need for some
customers having on-site oxygen plants such as a PSA oxygen
plant, for a small quantity of nitrogen which purity might be
different from one customer to another or which purity might
vary over the time on one site according to the customer's
need. For example, some customers need a source of nitrogen
of controllable purity or 4f variable flow rate, or both.
When a customer requires large amounts of oxygen, such as
in excess of about 15 tons per day or more, oxygen is usually
supplied on-site by an oxygen plant. At present, there are two
principal commercial types of oxygen plants in operation.
First, there are cryogenic plants, which utilize air li~uefac-
tion and distillation, and second, there are non-cryogenic
plants based on pressure swing adsorption (PSA), vacuum swing
adsorption (VSA), or a mixture of both. For a number of
classi~al oxygen applications, a small quantity of nitrogen is
also required for purge, transfer, blanketing, processing
and/or other purposes.
Although new oxygen cryogenic plants can also be designed
to produce a stream of nitrogen, the addition of an extra
nitrogen capacity would be an advantageous feature, particu-
larly if it can be at a different purity. However, existing
oxygen cryogeniG plants often have little or no nitrogen
capability. Moreover, oxygen plants based upon PSA or VSA, do
not allow for the recovery of a side nitrogen stream from the
adsorption process.
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Thus, in general, a need continues to exist for a means
of supplying nitrogen from any industrial plant having a need
therefor.
SUMMARY OF THE INVENTION
The invention essentially consists in oversizing the
compressor means provided in a new plant such as chemical plant
or air separation plant, when such means are provided, and
using the additional compressed air to feed a nitrogen membrane
to provide some on-site nitrogen product to fulfill the on-site
nitrogen r~quirements of the plants (or some additional on-site
requirements). For existing plants, which existing compressor
means can be boosted, or which compressor means need to be
changed for whatever reason, the additional compressed air
available is used to feed a nitrogen membrane. The invention
is particularly useful in those cases where there is a need of
a small quantity of nitrogen in the plant, particularly when
the requirements of nitrogen can be of different flowrate~
andtor purity at different periods of time. It is alss
particularly useful in cases where a plant is~connected to
pipelines providing nitrogen and/or oxygen, when there is an
additional need of nitrogen of di~ferent purity (e.g. low
purity compared to the nitrogen provided by pipeline), or when
only an oxygen pipeline exists and a small amount of nitrogen
is necessary, which purity might be different from one customer
to another or different during different periods of time for
the same customer9
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates an oxygen plant and its related
instrument air compressor and control system, according to the
prior art.
Figure 2 illustrates a basic embodiment of the present
invention based upon an oxygen plant, having a built-in
instrument air compressor which may be used in cryogenic, PSA
and/or VSA oxygen plants.
Figure 3 illustrates another embodiment of the present
invention based upon an oxygen plant, wherein connection to
compressed air may be before or after drying as indicated by
a dashed line.
Figure 4 illustrates another embodiment of the present
invention wherein the instrument air dryer is removed and is
replaced by the membrane separator, which is sized so as to
supply the requirements of the external nitrogen application(s)
and of the instrument gas stream.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a process is
provided for producing nitrogen in an economical manner from
an on-site plant, for example, an oxygen plant regardless of
the process by which the oxygen is produced.
The present invention is based on the fact that certain
plants have a need for small quankitias of nitrogen on a
regular basis or from time to time, which need is expensive to
be fulfilled, while at the same time there are already means
av~ilable on site to make compressed air, usually to drive
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instruments which are used to conduct the process of the plant.
The plant may be, for example, a chemical plant with an
instrument air compressor. The plant may also be an air
separation plant having an instrument air compressor. However,
even if the plant has an instrument air compressor, the main
compressor (or the instrument compressor) used is o~ersized to
also feed a nitrogen-producing membrane.
Where the on-site plant already produces nitrogen (e.g.,
a cryogenic air separation plant), the membrane can produce
nitrogen of a purity and/or pressure which is different from
that of the nitrogen produced by the plant.
According to one aspect, the present invention provides
a process for supplying nitrogen from an on-site plant, the
plant comprising compressor means and equipment means, at least
some of the equipment means being ~ed by compressed air from
the compressor means, said process comprising:
a) feeding an air stream to said compressor means which
is oversized relative to the compressed air needs to feed the
equipment means;
b) feeding at least a portion of the compressed air
stream from said compressor means to a nitrogen membrane
separator, thereby providing a stream of nitrogen product; and
c) venting an oxygen-enriched air-stream ~rom the
membrane.
Additionally, in the above process, the oxygen-enriched
air may be recoYered from the membrane instead of venting the
same.
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Further, the above process may entail the additional step
of feeding another membrane with the oxygen-enriched air stream
from the nitrogen membrane.
In accordance with this process, it is also advantageous
if the other membrane is a nitrogen membrane. It is, moreover,
also preferred if the compressor means is an instrument air
compressor and wherein the aquipment means are instrument means
dedicated to the control of the on-site plant.
In accordance with another aspect of the present inven-
tion, a process is provided for supplying nitrogen from an on-
site air separation plant providing at least oxygen product,
the plant comprising compressor means to compress air needed
for said separation plant, the process comprising the steps of:
a) feeding an air stream to the compressor means which
is oversized relative to the compressed air needs of the on-
site separation plant; and
b) feeding at least a portion of the compressed air
stream from the compressor means to at least one nitrogen
membrane separator, thereby providing a stream of nitrogen
product.
In accordance with this process, the air separation plant
can provide also nitrogen product and the stream of the
nitrogen product from the membrane has a different purity
and/or pressure than that provided by the air separation plant.
In yet another aspect thereof, the present invention
provides a process for supplying nitrogen from an on-site
oxygen plant, the oxygen plant comprising a separation plant
to separate oxygen from an oxygen-containinq gas mixture, such
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as air, an instrument air compressor and equipment means such
as valves or the like, actuated by compressed air from the
instrument air compressor, the process comprising:
a) feeding an air stream to the instrument air compres-
sor which is oversized relative to the compressed air needs
to actuate said equipment means, and
b) feeding at least a portion of the compressed air
stream from the instrument compressor to a nitrogen membrane
separator, thereby providing a stream of nitrogen product.
In accordance with any of these processes, the oxygen-
enriched air stream m~y ~e re¢overed from the membrane instead
of venting the same.
Additionally, the process may further entaii feeding
another membrane with the oxygen-enriched air stream from the
nitrogen membrane.
Further, it is advantageous if the other membrane is a
nitrogen membrane, and it is preferred if the compressor means
is an instrument air compressor and wherein the equipment means
are instrument means dedicated to the on-site plant control.
In any of the processes of the present invention, however,
an additional st~p may be used wherein the nitrogen product is
passed from the membrane separator to an oxygen-removing means,
such as a deoxo unit or a getter system, to deliver high-purity
nitrogen.
In accordance with any of the present processes, it is
preferred if the on-site plant i~ a cryogenic air separation
unit, a pressure swing adsorption unit or vacuum swing
adsorption unit, or any co~bination thereof.
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It is particularly advantageous, if pressure ~wing
adsorption or vacuum swing adsorption or both are used, to
further inject waste exhaust gas therefrom, which is enriched
in nitrogen gas, to the air instrument compressor intake. It
is, moreover, also advantageous to further recover the waste
oxygen-rich stream from the at least one membrane separator and
injecting the same at the inlet of one or more adsorbers of a
VSA unit.
For the specific application to on-site oxygen plants, the
present invention may be practiced in conjunction with any
conventional cryogenic, PSA or VSA process.
In a typical cryogenic process, for example, air is
compressed to a sufficient pressure in a compressor. A main
exchanger precools the air against effluent product streams and
extracts carbon dioxide and moisture through condensation and
solidification. An expander provides the process refrigera-
tion, while high-pressure and low-pressure columns separate the
oxygen and nitrogen primary components and concentrate argon.
A crude argon column may further separate argon from oxygen.
As air is cooled in the main exchanger, moisture and
carbon dioxide are condensed and solidified. At regular
intervals, the air passages and waste nitrogen vent passages
are switched so that the air flows through clean passages and
begins to build up a deposit of moisture and carbon dioxide.
At the same time, returning waste nitrogen revaporizes the
deposited water and carbon dioxide, thereby cleaning the
passage ways, ~or the next switrh. Alternatively, air can be
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purified from moiskure and carbon dioxide by means of a
regeneration adssrption process.
The double distillation column efficiently separates
oxygen and nitrogen, affording high recovery of components from
air.
For example, EP-A-081178 discloses an oxygen gas produc-
tion apparatus containing a single pressure distillation column
fed by an air compressor powered by a gas turbine, which in
turn is fed by pressurized gas coming from the combustion of
the oxygen containing nitrogen waste with fuel gas.
Adsorption based processes such as Pressure Swing
Adsorption and/or Vacuum Swing Adsorption, known respectively
as VSA and/or PSA processes, are based on a procedure in which
the gas content of the crude gas mixture which has the higher
affinity for the adsorbent is held on the surface of the
adsorbent in an adsorption step, and the less strongly adsorbed
components are stripped off from the adsorber filled with
adsorbent.
In PSA, desorption o~ th~ adsorbed phase is achieved by
reducing the pressure after the adsorption step and usually
with rinsing of the adsorben~ with some of the gas adsorbed
less strongly, and in particular under a pressure of 1 bar or
more. However, the desorption pressure may be reduced to a
pressure of less than 1 bar by means of a vacuum pump, and the
adsorbent is also rinsed with some of the gas adsorbed less
strongly. Processes using such vacuum desorption are VSA
processes.
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In a typical pressure swing adsorption process, for
example, utilizing the differential adsorption of nitrogen and
air impurities from oxygen on zeolites, a first unit of a
multiple bed system is pressurized, each of which contains two
strata, the first removing water and carbon dioxide, the second
adsorbing nitrogen from the flowing air. Oxygen product then
passes on to the product compressor if required. Depending
upon the operating cycle and conditions, the product may be up
to 95% oxygen by volume, with the balance argon and nitrogen.
A typical PSA process is disclosed in USP 3,866,428, incorpo-
rated herein as a reference.
Typical VSA processes are described in USP 3,493,296,
4,684,377 or 5,015,271, incorporated herein as references.
USP 5,015,271 discloses a two-adsorber VSA system which
requires only slightly higher energy consumption than the
three-adsorber VSA system, but which is more favorable in terms
of plant investment requirements due to the small number of
valves and adsorbers used. The disclosed process operates the
two adsorbers alternately so that the crude gas feed and the
desorption of the adsorbate is not stopped at any time during
the separation process, and separation of the crude gas partly
occurs under reduced pressure when the maximum adsorption
pressure has been reached.
In accordance with the present invention, and particularly
in using any cryogenic, PSA and/or VSA process, an instrument
air compressor is utilized which is oversized relative to the
required needs of the plant. ~owever, the compressor may be
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connected to the other components of the air separ~tion system
in a variety of ways.
For example, the membrane separator may be connected to
the instrument air compressor before the air dryer. In such
a case, only the external nitrogen application(s) will be
supplied by the membrane separator and the dry instrument air
supply necessary to the operation of the PSA or VSA will still
~e supplied from the instrument air dryer.
As another alternative, the membrane separator may be
connected after the instrument air dryer in order to benefit
from the purification of the air from such a system. However,
as in the previous arrangement, only the external nitrogen
application(s) will be supplied from the membrane separator.
Further, the instrument air dryer may be removed and
replaced by the membrane separator. This separator is sized
so as to supply the requirements of the external nitrogen
application(s) and of the instrument gas stream. This gas
stream is then not air, but a dry nitrogen stream from the
membrane separation.
Additionally, in accordance with the presPnt invention,
as the membrane separator is made of several membrane bundles,
it may be arranged into two or more bundle banks supplying
nitrogen at various purities and pressures for the external
application(s) and/or instrument gas requirements in order to
ensure the maximum cost-effectiveness of the overall system.
For example, the external application(s) may be at 99% nitrogen
purity, whereas the purity of the instrument gas may be at 90%.
As the purity of the nitrogen gas from the membrane system of
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a given permeation area is a function of the flowrate of intake
air, this means that less fibers ~rom the bundle are necessary
to make 90% purity nitrogen at the same flowrate than to make
99% purity nitrogen. Either the banks have the same permeation
areas, which means that less 99% than 90~ nitrogen will be
produced or they have different permeation areas, which means
that each flowrate can be adapted, including the same flowrates
for both.
Regarding the various possibilities of using a membrane
or several membranes arranged together, reference is hereby
made to the copending applications of Brugerolle at al. and
Barbe et al. which are incorporated herein as reference~, and
which were filed on June 3, 1991 and July 2, 1991 under Serial
Numbers 07/70~,226 and 07/725,773, respectively.
However, the present invention may be practiced in
conjunction with any cryogenic, PSA or VSA process using any
arrangement of elements with the oversized compressor.
In addition to the above illustrative embodiments, a
variety of other modifications may also be used.
For example, a purification unit such as a deoxo or getter
unit, may be installed downstream after the membrane separator,
so as to produce a high purity nitrogen stream. A deoxo unit
is a unit wherein usually some hydrogen is provided to have a
catalytic combustion with the oxygen present in the nitrogen
gas. A getter unit has the same deoxidation function using,
e.g., silane gas SiH4 in a small amount whi~h is a ~trong
reducing agent reacting with the oxygen under well-known
conditions.
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Additionally, the membrane unit may be installed indoors
in the same ~uilding as the cryogenic, PSA and/or VSA for every
weather protection and maintenance.
Moreover, the membrane unit may be operated with the same
computer, programmable logic controller or controls, as the
rest of the cryogenic, PSA and/or YSA.
Further, the instrument air intake and the waste exhaust
gas of the PSA and/or VSA may be connected. This allows the
feeding of a gas already enriched in nitrogen to the membrane
separator instead of straight air. An intermediate buffer tank
with adequate condensate purges may be required.
Also, the waste oxygen rich stream gas from the membrane
separator may be recovered and injected at the inlet of the PSA
and/or VSA adsorbers, either upstream or downstream of the PSA
air compressor or VSA air blower.
In order to further illustrate the present invention,
Figures 1-4 will now be described in more detailO
Figure 1 illustrates the basic features, for the purpose
of the present invention, of an oxygen plant according to the
prior art. The separation plant which produces oxygen product
can be of any type (PSA, VSA, cryogenics, etc.) as previously
disclosed and is controlled by instrument means which are
valves, actuators, etc. Those instr~ment means are pneumati-
cally controlled means which need compressed air (or gas) to
actuate them. The plant thus comprises an instrllment air ~or
gas) compressor feeding the instrument means through e.g. a
prefilter, an instrument air dryer and an after filter. The
instrument air compressor is sized to the needs of compressed
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air of the instrument means.
Figure 2 represents one embodiment of the invention in
case of an oxygen plant. The oxygen plant is basically
identical to that of ~igure 1 (even if prefilter, dryers and
after filter have not been represented). But the instrument
air compressor is oversized compared to that of Figure 1, in
such a way that said oversized compressor is also able to feed
a nitrogen membrane system made of a bundle, of hollow fibers,
usually made of polymer such as polyamide, polyimide, polysulf-
ane, as the like, said bundle being located in sealed case, the
compressed air feeding the outside of the fibers while oxygen
preferably permeates through the wall of the hnllow fiber
(permeate gas) and nitrogen do~s not permeate (non~permeate
gas). The non-permeate gas, which can be nitrogen having an
excellent purity, is recovered as a product sent to tha user
while the permeate, which is oxygen-enriched gas (compared to
air), is vented (waste gas) or might be recovered and used as
a by-product.
Figure 3 illustrates an embodiment of the invention which
is similar to Figure 2 except that the compressed air which
feeds the membrane is either taken at the output of the
prefilter and can thus contaln some moisture or after drying
and another filtexing when dry air is necessary, which is
better for the membrane (wet air might be detrimental to the
membrane).
Yigure 4 illustrates the case where the instrument means
are actuated by the compressed nitrogen from the membrane
system. After prefiltering, the compres~ed air is fed to the
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membrane and the nitrogen product is used both b~ the user and
after filtering to actuate the instrument means. Optionally,
the waste oxygen enriched gas is recovered to feed the air
separation plant, particularly in the case of a PSA/VSA oxygen
unit where thi~ can improve the overall yield of the system.
For each sf the Figures 2 to 4, the temperature and/or
pressure of the membrane system may be controlled in accordance
with the copending application of Brugerolle et al., filed June
3, 1991, S.N. 07/709,226, which application is incorporated
herein by reference, which can in some cases allow to generate
high purity nitrogen such as required in electronics, with an
acceptable flow rate.
Moreover, for the apparatus of each figure, the membrane
separator may be a cascade system as described in the copending
application Barbe et al., filed July 2, 1991, S.N. 07/725,773,
which application is incorporated herein by reference.
However, any type of membrane cascade system providing
different nitrogen purities may be used, such as low purity for
the instrument means and higher ~urity for the user.
Additionally, even though it is preferred to use a
membrane to produce nitrogen product, a nitrogen PSA may be
used instead o~ the membrane, while ~till being within the
scope o~ the present invention.
Having described the present invention, it will now be
apparent to one o ordinary skill in the art that many changes
and modifications can be made to khe above-described embodi-
ments without departing from the spirit or the scope of the
present invention~
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